RTV silicones have long been used for gasketing applications in which a liquid resin is placed between two mating parts of a fluid conduit system, the parts mated to extrude the gasketing material over the mating surfaces and then the formulation allowed to cure. The flexibility of the resulting gasket is highly suited to the sealing requirements of mated parts, particularly parts of different materials with different coefficients of thermal expansion. Such RTV cured-after-assembly silicones have also been formulated which have good resistance of a wide variety of hostile environments.
RTV silicones have frequently been formulated as automotive engine cured- after-assembly gasketing materials where high temperature aggressive fluids, such as engine coolant and motor oil, created a particularly hostile environment. Examples of silicones formulated for such automotive gasketing applications are described in U.S. Pat. Nos. 4,673,750; 4,735,979 and 4,847,396 to Beers et al; and co-pending application Ser. No. 07/607,571 filed Nov. 1, 1990.
An important part of the environment resistance which these formulations achieve is due to the strong silicone/substrate adhesion developed by particular adhesion promoters in these formulations. However, where removability of the gasket is important, e.g. where gasket replacement may be necessary for maintenance or repair of an assembly, such strong substrate adhesion makes the use of such cured-after-assembly gaskets undesirable. Moreover, cured-after-assembly gaskets may create problems when an excessive amount of RTV is used because uncured fillets may be extruded into fluid passageways where they may contaminate fluids or cure so as to partially or completely block a passageway.
In contrast to liquid cured-after-assembly gaskets, it is also known to utilize preformed compression gaskets as seals between automotive engine parts. Compression gaskets are easily removed and replaced but suffer the disadvantage of an inventory of gaskets must be maintained for each configuration of mating parts. Further, "compression set" is a serious problem with preformed gasket materials. "Compression set" is a loss of resiliency of the gasket material over time as the gasket is maintained under pressure and subjected to high temperature. Sealing function may be irrecoverably lost if pressure is lessened or if the assembly is subjected to relative movement, e.g., because of differential thermal expansion of the mating parts. Common materials used in compression gaskets include soft metals, corks, cork composites and various crosslinked rubbers. Certain heat crosslinked silicone rubbers have also been used. Such rubbers typically have a very high tensile modulus under 100% elongation so that under compression they do not have the flexibility to conform well to uneven surfaces, particularly on mating parts of different materials having different coefficients of thermal expansion.
UV-curable silicones, such as disclosed in U.S. Pat. No. 4,675,346, have been proposed for formulation of formed-on-part compression gaskets which are cured before mating, but such silicones are only practical in assembly operations where UV ovens are available. They are not practical for use as maintenance and repair gaskets and have not found use in automotive engine applications.
Another type of gasketing application has been proposed as a hybrid of the cured-after-assembly and the preformed compression gasket. A liquid resin is preapplied to one of two mating parts as a continuous bead and the second part mated prior to cure. After cure, the bead is compressed. Such a formed-on-part compression gasket has the advantage of eliminating inventory requirements for conventional gaskets and allowing rapid production of custom gaskets, but it introduces a cure time lag in the assembly operation. Moreover, the cured bead material has potentially all the other problems of the conventional compression gaskets.
There is, therefore, a need for a formed-on-part compression gasket material suitable for use as a replacement for conventional pre-formed gaskets in automotive engine application. In particular, the material should be a liquid resin formulation which has a very fast ambient cure time, provides resistance to automotive fluids, is readily removable, and has low compression set. Prior art RTV silicones do not have a suitable combination of these properties.